One problem often encountered in the production of natural gas from underground reservoirs is nitrogen contamination. The nitrogen may be naturally occurring and/or may have been injected into the reservoir as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation. Natural gases which contain a significant amount of nitrogen may not be saleable, since they do not meet minimum heating value specifications and/or exceed maximum inert content requirements. As a result, the feed gas will generally undergo processing, wherein heavier components such as natural gas liquids are initially removed, and then the remaining stream containing primarily nitrogen and methane is separated cryogenically. A common process for separation of nitrogen from natural gas employs a single column or a double column distillation cycle wherein the feed is separated into a nitrogen-enriched vapor and methane-enriched liquid.
Liquid nitrogen is a desirable product in that it may be employed to provide refrigeration for a process such as a freezing process, or may be stored for subsequent vaporization and use for inerting, nitrogenation or other purposes. The nitrogen generated as a result of a hydrocarbon nitrogen rejection operation is a convenient source of nitrogen. However, production and recovery of nitrogen as liquid is costly because considerable additional equipment is required to use excess refrigeration in the process to condense nitrogen without upsetting the stability and separation efficiency of the process.
Accordingly it is an object of this invention to provide a system for the production of liquid nitrogen which is effectively employed in conjunction with a hydrocarbon processing system using a nitrogen rejection unit.